Ill] ENZYME ACTION 19 



The chemical nature of enzymes is at ])resent unknown, because it 

 is difficult to purify them without destroying them, and hence to obtain 

 them of sufficient purity for chemical analysis. They were originally 

 thought t(^ be proteins, but with the improvements in methods for 

 purification, it has been found that the protein reactions disappear, 

 although the enzyme activity does not decrease. In solution they exist 

 in the colloidal condition. 



The questions as to their origin and their relation to the protoplasm 

 cannot yet be answered with any certainty. It is also impossible to say 

 whether the majority of chemical processes in the plant are catalyzed by 

 enzymes. 



A feature of enzyme action which is of considerable interest and 

 which has already been mentioned is the question as to whether enzymes 

 catalyze the synthetic as well as the hydrolytic reaction. There is 

 evidence that this is so, since, in many cases, the hydrolysis is not 

 complete. If the enzyme were a catalyst in one direction only, the 

 reaction would be complete. Further evidence is supplied by the fact 

 that, under suitable conditions, i.e. strong concentration of the substances 

 from which synthesis is to take place, certain syntheses have been carried 

 out in vitro. As an example may be quoted the synthesis of maltose 

 from a concentrated solution of glucose by maltase (Bayliss, 2). 



In the living cell it is supposed that the hydrolysis and synthesis 

 are balanced. On the " death " of the protoplasm, which may be caused 

 by mechanical injury, vapour of chloroform or toluol, etc. (Armstrong, 7, 8), 

 the reactions catalyzed by enzymes cease to be balanced and proceed almost 

 always in the direction of hydrolysis and the splitting up of more complex 

 into simpler substances. This phenomenon is obvious when any of the 

 products can be recognized by smell or colour, as, for instance, the smell 

 of benzaldehyde on injuring leaves of plants containing cyanogenetic 

 glucosides (see p. 146), or the production of coloured oxidation products 

 when some of the aromatic glucosides are decomposed (see p. 113). 



If plant tissuesare disintegrated, and the mass is kept at a temperature 

 of about 88° C, the above-mentioned hydrolytic processes continue to be 

 catalyzed by the enzymes present until equilibrium is reached, which will 

 be near complete hydrolysis, especially if water has been added. Such 

 a process is termed "autolysis." 



The chief plant enzymes may be classified under the following- 

 headings : 



A. Hydrolytic enzymes. These constitute by far the greater number 

 of known enzymes. In their activity they either add or eliminate water. 



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